The present invention relates to measuring the speed of an aircraft by means of an ultrasonic transmitter and appropriately placed receivers.
The measurement of aircraft speed by means of ultrasonics is known generally. For this, a transmitter is suitably placed on the aircraft and two receivers are placed in foreward and aft direction from the transmitter i.e. in opposite directions along a line that runs parallel to the longitudinal center line or axis of the aircraft and at equal spacings from the transmitter. Due to the air flow along the craft a transmitted ultrasonic signal will arrive at the two receivers at different times. The transit time differential can be used directly to determine the speed of the craft relative to the ambient air.
In order to practice this method, it is customary to transmit the ultrasonic waves in the form of pulses, and transit time differences are determined on the basis of the pulse arrival times. Relatively high aircraft speeds can be measured in this manner quite accurately, particularly because transient effects at the respective pulse edges are negligible in relation to the detectible transit time differences. The situation is different in the case of medium or low speeds of e.g. VSTOL type aircraft when in a hovering phase at a rather low propagation speed. The initial transient phase at the onset of a pulse may falsify the measuring result up to by 20% or even more. For this reason, one has deemed this ultrasonic speed measurement as unsuitable for vertical take off and landing crafts.